This invention relates to wood preservatives, and more particularly, it is related to a wood preservative composition comprising a metal salt and a graft copolymer of lignosulfonate.
In the United States, as in many other countries possessed of abundant timber resources, wood has always been an important construction material. Its good availability, great strength, ease of shaping and fastening, low heat conductivity, and sound-deadening qualities, have made it the outstanding building material since early human history. However, two major destructive forces of wood are fire and decay. While fire is often caused by human factors, decay is due to biological attacks by microorganisms (such as fungi and bacteria), marine borers and insects. Much of the decay of wood in service is inevitable. It is particularly severe as a result of the activities of low forms of plants known as wood-destroying fungi. This type of fungi often causes extensive damage and financial loss to buildings in the temperate zone.
The conditions necessary for the development of decay-producing fungi in wood are: (1) a supply of suitable wood, (2) a sufficient amount of moisture, (3) at least a small amount of air, and (4) a favorable temperature. A deficiency in any of these requirements will inhibit the growth of a fungus, and reduce the decay of wood. Under normal service conditions of wood, the deficiency is difficult to maintain. So, a more common method for preserving wood against the fungal attacks is to impregnate it with certain wood preservatives. There are two types of wood preservatives. Oil-borne preservatives include creosote, pentachlorophenol, copper naphthenate, zinc naphthenate, copper-8-quinolinolate and others. Important water-borne preservatives are arsenic salts, boric acid, chromium salts, chromated zinc chloride, copper sulfate, mercuric chloride, sodium pentachlorophenate, zinc sulfate, chromated copper arsenate, etc. For more details on the above-mentioned preservatives, see Hunt and Garratt, Wood Preservatives, McGraw-Hill, New York, 1953.
According to a marketing study (Forest Industries, November 1985, pp. 36-38), chromated copper arsenate (or CCA) is the current most widely used lumber preservative. About 95% of treated lumber (a total of 4 billion board feet in 1984) is treated with CCA, a water-borne preservative composed of metallic salts. CCA gives the lumber the best available protection against decay and insects. Southern yellow pine from southern lumber mills represents almost three-quarters of treated lumber. The rest is treated yellow pine lumber in the Northeast and treated ponderosa pine in the Midwest. Wood preservatives such as those described above have been applied to the wood as solutions, emulsions, pastes or dispersions in liquid hydrocarbons and/or aqueous systems. In many applications, aqueous systems are preferred over liquid hydrocarbons because of the odors, flammability and often toxic nature of the hydrocarbon products. In contrast, water-borne preservatives tend to be less odorous, non-flammable and give a clear appearance to the treated wood. Yet, the aqueous preservative compositions often contain chromium and arsenate that are considered to constitute a significant risk to human health and the environment. For these and other reasons, a need exists for low-cost, non-hazardous, water-borne preservatives that contain no toxic chemicals. A common approach to achieving the object in the prior art involves fixation of certain metals such as zinc, copper, mercury, antimony, lead and so on in the wood with organic chemical compounds. For example, U.S. Pat. No. 4,783,221 (Nov. 8, 1988) teaches the use of a preservative composition comprising a metal salt of organic carboxylic acid of six to about 30 carbon atoms and an isothiazolone compound in preserving wood. Swedish Patent No. SE 430865 B (Dec. 19, 1983) reveals a preservative composition containing copper or zinc salt of acetic and propionic acid. Australian Patent AU 519416 B2 (Nov. 12, 1981) discloses a mixture of copper oxide, decanoic acid and ethanolamine. Canadian Patent No. CA 978474 (Nov. 25, 1975) claims the use of zinc or copper salt of fatty acids as wood preservatives. And disclosed in Belgian Patent BE 903306 A4 (Jan. 16, 1986) is a wood preservative consisting of 87 parts of citric acid, 100 parts of copper carbonate and 446 parts of 30% NH.sub.3 that is claimed to improve the resistance of pine wood to attacks by fungi and insects.
Quite recently, in European Patent Application No. 86850112.3 (Apr. 1, 1986), a method for the impregnation of wood is revealed for preventing the attack by fungi and bacteria. The method comprises the application of an aqueous solution of an alkali lignin modified into a water-soluble form, characterized in that the impregnation takes place in two stages, these being an initial stage in which the aqueous solution containing the lignin, still in its water-soluble form and with a pH not exceeding 10, is applied to the timber (wood) in order to be absorbed by it, and a second stage in which the lignin is fixed by being transformed into an essentially water-insoluble form in order to be retained in the timber, said second stage involving the application of a weakly acidic aqueous solution containing a metal salt or a combination of metal salts, said metal being zinc, copper or aluminum. The most common alkali lignin is the kraft lignin (or sulfate lignin) produced in the pulping of wood by the kraft process. The other alkali lignin is the lignin produced in the soda pulping process (soda lignin, currently not available commercially). Alkali lignin is generally water-insoluble and may be rendered water-soluble by certain carboxylation and sulfonation reactions as are practiced in EP 86850112.3. The nature of alkali lignin is such that in a mildly carboxylated and/or sulfonated form, it is water-soluble at neutral or slightly alkaline pH, but becomes insolubilized at an acidic pH. The so-called water-soluble modified alkali lignin also tends to be "salted-out" by metal salts. This is the principle of fixation of lignin (with weakly acidic solution of metal salts) taught by EP 86850112.3.
The other major type of lignin produced by the paper industry is sulfite lignin (or lignosulfonate) from pulping of wood by the sulfite process Lignosulfonate is water-soluble at all pH's, including acidic, neutral and alkaline pH's. In general, it cannot be insolubilized by addition of mineral acids or metal salts. Therefore, lignosulfonate as derived normally from the sulfite pulping liquors cannot be "fixed" into the wood by the two-step method of said European Patent Application No. 86850112.3. Lignosulfonate absorbed into the wood that is subsequently treated with a metal salt such as copper and zinc salts, is easily leached out of the wood by water and loses its effectiveness as a wood preservative.
Currently, only two types of commercial lignins are available in the marketplace, namely, kraft lignin and lignosulfonate. The production capacity of lignosulfonate is estimated at 1,340 million kilograms per annum in the Western World and that of kraft lignin is only about 45 million kilograms per annum. The unproportionally greater quantity of commercially available lignosulfonate makes it attractive and desirable to produce a wood preservative composition based on sulfite lignin. Furthermore, it will be advantageous to have a lignin-based wood preservative that can be applied to the wood by a one-step method, instead of the two-step method taught in the prior art.
It is thus an object of the invention to provide a water-borne preservative for protecting wood against microorganism attacks.
It is a further object of the invention to provide a wood preservative that is based on lignosulfonate from spent sulfite pulping liquors.
It is an additional object of the invention to provide a low-cost wood preservative from the solids of spent sulfite pulping liquor.
It is another object of the invention to provide a lignosulfonate-based preservative that can be applied to the wood by a simple, one-step impregnation method.
It is a specific object of the invention to provide a wood preservative that is essentially non-toxic.